1. Field of the Invention
This invention relates generally to an image forming apparatus which uses rotating multifaceted reflective polygon scanners. In particular, this invention relates to a method for correcting facet-to-facet pyramidal wobble using agile beam concepts and all-spherical optics in place of conventional cylindrical optics.
2. Related Art
Facet-to-facet pyramidal wobble arises in any image forming apparatus which uses a rotating multifaceted reflective polygon scanner. While each facet of the polygon is ideally perfectly parallel to the axis of the rotation of the polygon, producing such a polygon scanner is generally impossible using standard manufacturing techniques. In general, each facet will be slightly skewed in the axial direction relative to the rotational axis of the polygon. Further, each facet will be skewed to a different degree. Without correcting for such facet-to-facet wobble, an image forming apparatus using a rotating multifaceted reflective polygon would form each scan line above or below the desired position. The amount of positive (above) or negative (below) displacement depends on the degree and sign (positive or negative) of the skew angle formed between the facet and the rotational axis of the polygon.
Conventionally, facet-to-facet pyramidal wobble has been corrected by using cylindrical optical elements in the pre-polygon and post-polygon optical systems. However, cylindrical optical elements are both expensive to manufacture and difficult and time consuming to accurately position within the image forming apparatus.
In contrast, spherical optical elements are both inexpensive to manufacture and relatively easy to install in the image forming apparatus. However, spherical optical elements are unable to inherently correct for wobble, as is the case with cylindrical optical elements. Accordingly, if all spherical optical elements are to be used, some additional mechanism which is capable of correcting facet-to-facet pyramidal wobble must be incorporated into the image forming apparatus.
U.S. Pat. Nos. 5,204,523, 5,208,456 and 5,212,381 to Appel and Paoli, who are also two of the inventors of this invention, describe "agile beam" concepts, and are incorporated herein by reference. As described in these patents, "agile beam" correction involves a tunable wavelength diode laser or other variable wavelength light sources or a variable index of refraction prism or similarly tunable electrical optical device. By using a variable wavelength light source or a variable index of refraction prism, slow scan direction process errors can be compensated for. These errors include ROS-ROS misregistration errors, photoreceptor motion errors or photoreceptor surface defects.
In the devices and methods described in these references, determination of slow scan direction positional errors in a previous scan line can be measured and used to alter either the variable wavelength of the light source or the variable index of refraction of the prism to compensate for the positional error of a current scan line. However, such post-formation correction is inapplicable to the problem of facet-to-facet pyramidal wobble, as the positional errors associated with each individual facet change from facet to facet. In contrast, the types of errors corrected in the previous agile beam devices are generally stable and repeatable. Thus, the previous agile beam devices and methods are insufficient to deal with facet-to-facet pyramidal wobble.